1. Field of the Invention
The present invention relates to a laser receiver and a laser receiving system, and in particular to a laser receiver receiving a laser beam from a laser transmitter and a laser receiving system composed of a plurality of laser receivers.
2. Description of the Related Art
Some conventional laser receivers have a single photo device (photoreceptor device or receiving optics) while others have a plurality of photo devices. Any of the laser receivers has the photo devices arranged therein so that a directivity and an acceptance sensitivity suitable for the purposes of the systems may be achieved in consideration of the directivities of the photo devices.
FIG. 29 shows directivity of a general photo device. When a photo device 10 is arranged on an acceptance surface L as shown in FIG. 29, the acceptance sensitivity in a vertical direction (zenithal direction) with respect to the acceptance surface L is maximum and the acceptance sensitivity decreases as an angle (elevation angle) is lowered from the zenith. As shown in FIG. 29, when the acceptance sensitivity in the zenithal direction is supposed to be 100%, the acceptance sensitivity near a horizontal direction to the acceptance surface L assumes the order of 30%.
Thus, general photo devices have the maximum acceptance sensitivity in the zenithal direction with respect to the acceptance surface, the acceptance sensitivity thereof decreasing as the angle is lowered from the zenith.
FIGS. 30A and 30B show an example of a single photo device arranged on a conventional laser receiver. FIG. 30A is a plan view of a laser receiver 20, and FIG. 30B is a side view thereof. The laser receiver 20 shown in FIGS. 30A and 30B has a photo device 21 with a large acceptance area arranged on the acceptance surface L. The acceptance sensitivity of this laser receiver 20 is directly affected by the directivity of the photo device 21, so that the acceptance sensitivity is maximum in the zenithal direction, decreasing as the angle is lowered from the zenith. Therefore, it has not been possible to accept a laser beam from the horizontal direction with respect to the acceptance surface L.
FIG. 31 is a circuit arrangement of the laser receiver 20 shown in FIGS. 30A and 30B. As shown in FIG. 31, the laser receiver 20 is composed of the photo device 21, a level detection circuit 30 and a controller 33. The level detection circuit 30 has an amplifier 31 for amplifying a signal accepted (received) by the photo device 21, and comparators 32_H and 32_L for comparing the signal amplified by the amplifier 31 with reference voltages VH and VL, respectively.
It is supposed that there is a relationship of VH>VL between the reference voltages VH and VL. Therefore, the comparator 32_H detects a signal of a higher reference, voltage. The outputs of the comparators 32_H and 32_L are provided to the controller 33 as signals 301 and 302, respectively.
Thus, the level detection circuit 30 has been able to shape the signal received by the photo device 21 and obtain information indicating a level at which the laser beam is received by using the comparators 32_H and 32_L.
Thus, the laser receiver having a single photo device has been able to detect a level of the laser beam received. However, even if the center of the laser beam hits a position off the laser receiver, it has not been able to detect how off the center of the laser beam hits the laser receiver. Also, it has not been able to detect an acceptance angle at which the laser beam is received.
On the other hand, as an example of a laser receiver having a plurality of photo devices, Japanese examined patent application publication No. 7-26808 discloses a method of detecting an acceptance position of a pulse laser light by arranging numerous photo devices all over the acceptance surface.
By this method, it is made possible to detect the acceptance position of the laser beam within the area where the photo devices are arranged. However, since it is required to arrange numerous photo devices to the entire area where the acceptance position is to be detected, a massive number of photo devices are required for the detection of the acceptance position of the laser beam within a large area.
Also, it has not been able to detect an acceptance angle at which the laser beam is received by this method.
In a communication system performing data transmission/reception by using a laser beam, it is usually required to perform an optical axis adjustment between a laser transmitter and a laser receiver before starting the data transmission/reception.
It is ideal that the center of the laser beam from the laser transmitter hits the center of the laser receiver vertically. Therefore, if the laser receiver can detect the distance between the center of the received laser beam and the center of the laser receiver as well as the acceptance angle, it is made possible to perform an optical axis adjustment by using the results thereof.
Also, the laser receiver can be used as a target for determining whether a laser beam as a dummy bullet hits or misses in e.g. a simulant target training system.
In this case, if the acceptance position of the center point of the received laser beam on the acceptance surface is detected by using the conventional laser receiver, it is required to arrange numerous laser receivers in order to cover the surface of the target object in case the target is a large object such as a human body. Also in case of the laser receiver having a plurality of photo devices, it is required to arrange a massive number of photo devices in order to cover the surface of the target object, so that it is unrealistic.